Elevation in the number of CD8+ cells is a consistent feature of HIV and FIV infections. In the case of HIV, this abnormal homeostasis is marked by a progressive loss of L-selectin (LS) positive naive CD8+ cells and a concomitant expansion of LS negative CD8+ effector cells, such that the CD8+ LSneg cells may represent 80-90 percent of the total circulating CD8+ cells in the late asymptomatic stage infection. FIV infection induces the progressive expansion of a CD8+ subset characterized by a marked reduction of the beta chain. This CD8+ betalo phenotype may comprise as much as 90 percent of total blood CD8+ cells at late-stage asymptomatic infection. We have taken advantage of beta chain down-regulation to FACStar sort highly enriched CD8+ betalo and CD8+ betahi phenotypes, and have shown that the CD8+ betalo phenotype synthesizes high levels of IL10 and IFNgamma mRNA, and has potent anti- FIV activity. In addition, we have shown by 2- and 3-color FACS that the CD8+ betalo cells is an effector phenotype (CD8+ betaloLSneg CD44hi) and the CD8+ betahi is a naive phenotype (CD8+ betahiLSneg CD44hi). This proposal will test the hypothesis that FIV induces a chronic expansion of a CD8+betaloLSneg CD44hi effector phenotype that not only has potent antiviral activity but exhibits abnormal tissue trafficking and immunosuppressive responses to secondary infections. Experiments will be designed to further characterize the phenotype of this CD8+betaloLSneg subset and its distribution in lymphoid and nonlymphoid tissues at different stages of FIV infection. Fluorescent dye labeling will be done to assess the trafficking potential of CD8+ betaloLSneg cells between blood and LN, and between blood and the lung in response to T. gondii infection. RT-qcPCR studies will be designed to determine if the CD8+betaloLSneg phenotype is regulated at the level of gene transcription and if it is related to virus load. Purified subsets of CD8+betaloLSneg (effector) and CD8+betahiLSpos (naive) will be assayed by RT-qcPCR assays for cytokine and chemokine mRNA to address the hypothesis that the CD8+ betaloLSneg has the cytokine/chemokine profile of a CTL and/or T suppressor cell. In vitro experiments will determine if CD8+ betaloSneg cells are CTL's or virus suppressor cells and whether they suppress mitogen and antigen-specific recall responses by PBMC. Studies will also address the hypothesis that CD8+betaloLSneg cells exert immunosuppressive effects on lung macrophages resulting in decreased cytokine responses and uncontrolled replication of T. gondii tachyzoites. These experiments will collectively test the hypothesis that FIV induces the chronic expansion of a novel CD8+betaloLSneg T- suppressor-like activation phenotype that not only mediates immunity to FIV, but because of its altered tissue trafficking (loss of L-selectin) and cytokine expression profile (e.g. IL10) is selectively recruited into inflammatory sites and suppresses immune responses to secondary pathogens.